The critical role of the boreal summer mean state in the development of the IOD

Boreal summer is a critical season for the rapid development of the Indian Ocean Dipole (IOD). In this study, three factors related to the boreal summer mean state are proposed to be important for the rapid development of the IOD, by strengthening the equatorial zonal wind anomaly and thus the dynamic Bjerknes feedback. Firstly, as part of the Indo-Pacific warm pool, the high mean SST in the southeastern tropical Indian Ocean (SEIO) acts as an essential prerequisite for the development of anomalous convection. Secondly, the maximum of the suppressed precipitation in response to a cold SST anomaly (SSTA) in the SEIO, shifts northward towards the equator because the mean precipitation is equatorially trapped in boreal summer. Thirdly, the monsoonal easterly shear in boreal summer promotes an enhanced, more equatorially symmetric low-level Rossby wave response to a prescribed equatorially asymmetric heating over the SEIO. The above three processes promote a greater equatorial zonal wind response and thus a greater Bjerknes feedback, as well as a greater IOD development during boreal summer.